Research Article Coupled Fixed Point Theorems for a Pair of Weakly Compatible Maps along with CLRg Property in Fuzzy Metric Spaces

Transcription

1 Applied Mathematics Volume 2012, Article ID , 13 pages doi: /2012/ Research Article Coupled Fixed Point Theorems for a Pair of Weakly Compatible Maps along with CLRg Property in Fuzzy Metric Spaces Manish Jain, 1 Kenan Tas, 2 Sanjay Kumar, 3 and Neetu Gupta 4 1 Department of Mathematics, Ahir College, Rewari , India 2 Department of Mathematics and Computer Science, Cankaya University, Ankara, Turkey 3 Department of Mathematics, DCRUST, Murthal, Sonepat , India 4 HAS Department, YMCAUST, Faridabad, India Correspondence should be addressed to Kenan Tas, kenan@cankaya.edu.tr Received 26 May 2012; Revised 6 July 2012; Accepted 8 July 2012 Academic Editor: Yansheng Liu Copyright q 2012 Manish Jain et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The aim of this paper is to extend the notions of E.A. property and CLRg property for coupled mappings and use these notions to generalize the recent results of Xin-Qi Hu The main result is supported by a suitable example. 1. Introduction and Preliminaries The concept of fuzzy set was introduced by Zadeh 1 and after his work there has been a great endeavor to obtain fuzzy analogues of classical theories. This problem has been searched by many authors from different points of view. In 1994, George and Veeramani 2 introduced and studied the notion of fuzzy metric space and defined a Hausdorff topology on this fuzzy metric space. Bhaskar and Lakshmikantham 3 introduced the notion of coupled fixed points and proved some coupled fixed point results in partially ordered metric spaces. The work 3 was illustrated by proving the existence and uniqueness of the solution for a periodic boundary value problem. These results were further extended and generalized by Lakshmikantham and Cirić 4 to coupled coincidence and coupled common fixed point results for nonlinear contractions in partially ordered metric spaces. Sedghi et al. 5 proved some coupled fixed point theorems under contractive conditions in fuzzy metric spaces. The results proved by Fang 6 for compatible and weakly

2 2 Applied Mathematics compatible mappings under φ-contractive conditions in Menger spaces that provide a tool to Hu 7 for proving fixed points results for coupled mappings and these results are the genuine generalization of the result of 5. Aamri and Moutawakil 8 introduced the concept of E.A. property in a metric space. Recently, Sintunavarat and Kuman 9 introduced a new concept of CLRg. The importance of CLRg property ensures that one does not require the closeness of range subspaces. In this paper, we give the concept of E.A. property and CLRg property for coupled mappings and prove a result which provides a generalization of the result of Preliminaries Before we give our main result, we need the following preliminaries. Definition 2.1 see 1. A fuzzy set A in X is a function with domain X and values in 0, 1. Definition 2.2 see 10. A binary operation : 0, 1 0, 1 0, 1 is continuous t-norm, if 0, 1, is a topological abelian monoid with unit 1 such that a b c d whenever a c and b d for all a, b, c, d 0, 1. Some examples are below: i a, b ab, ii a, b min a, b. Definition 2.3 see 11. Letsup t 0,1 Δ t, t 1. A t-norm Δ is said to be of H-type if the family of functions {Δ m t } m 1 is equicontinuous at t 1, where Δ 1 t t, Δ Δ m Δ m 1 t t. 2.1 A t-norm Δ is an H-type t-norm if and only if for any λ 0, 1, there exists δ λ 0, 1 such that Δ m t > 1 λ for all m N, when t> 1 δ. The t-norm Δ M min is an example of t-norm, of H-type. Definition 2.4 see 2. The 3-tuple X, M, is said to be a fuzzy metric space if X is an arbitrary set, is a continuous t-norm and M is a fuzzy set on X 2 0, satisfying the following conditions: FM-1 M x, y, 0 > 0 for all x, y X, FM-2 M x, y, t 1ifand only if x y, for all x, y X and t>0, FM-3 M x, y, t M y, x, t for all x, y X and t>0, FM-4 M x, y, t M y, z, s M x, z, t s for all x, y, z X and t, s > 0, FM-5 M x, y, : 0, 0, 1 is continuous for all x, y X. In present paper, we consider M to be fuzzy metric space with, the following condition: FM-6 lim t M x, y, t 1, for all x, y X and t>0. Definition 2.5 see 2. Let X, M, be a fuzzy metric space. A sequence {x n } X is said to be:

3 Applied Mathematics 3 i convergent to a point x X, if for all t>0, lim M x n,x,t 1, 2.2 ii a Cauchy sequence, if for all t>0andp>0, lim M x n p,x n,t A fuzzy metric space X, M, is said to be complete if and only if every Cauchy sequence in X is convergent. We note that M x, y, is nondecreasing for all x, y X. Lemma 2.6 see 12. Let x n x and y n y, then for all t>0: i lim M x n,y n,t M x, y, t, ii lim M x n,y n,t M x, y, t if M x, y, t is continuous. Definition 2.7 see 7. Define Φ {φ : R R }, and each φ Φ satisfies the following conditions: φ-1 φ is nondecreasing; φ-2 φ is upper semicontinuous from the right; φ-3 n 0 φn t < for all t>0, where φ n 1 t φ φ n t,n N. Clearly, if φ Φ, then φ t <tfor all t>0. Definition 2.8 see 4. An element x, y X X is called: i a coupled fixed point of the mapping f : X X X if f x, y x, f y, x y, ii a coupled coincidence point of the mappings f : X X X and g : X X if f x, y g x, f y, x g y, iii a common coupled fixed point of the mappings f : X X X and g : X X if x f x, y g x, y f y, x g y. Definition 2.9 see 6. An element x X is called a common fixed point of the mappings f : X X X and g : X X if x f x, x g x. Definition 2.10 see 6. The mappings f : X X X and g : X X are called: i commutative if gf x, y f gx,gy for all x, y X, ii compatible if lim M gf x n,y n,f g xn,g y n,t 1, lim M gf y n,x n,f g yn,g xn,t 1, 2.4 for all t>0 whenever {x n } and {y n } are sequences in X, such that lim f x n, y n lim g x n x, and lim f y n,x n lim g y n y, for some x, y X.

4 4 Applied Mathematics Definition 2.11 see 13. The maps f : X X X and g : X X are called w-compatible if gf x, y f gx,gy whenever f x, y g x,f y, x g y. We note that the maps f : X X X and g : X X are called weakly compatible if f x, y g x, f y, x g y, 2.5 implies gf x, y f gx,gy, gf y, x f gy,gx, for all x, y X. There exist pair of mappings that are neither compatible nor weakly compatible, as shown in the following example. Example Let X, M, be a fuzzy metric space, being a continuous norm with X 0, 1. Define M x, y, t t/ t x y for all t>0, x, y X. Also define the maps f : X X X and g : X X by f x, y x 2 /2 y 2 /2 and g x x/2, respectively. Note that 0, 0 is the coupled coincidence point of f and g in X. It is clear that the pair f, g is weakly compatible on X. We next show that the pair f, g is not compatible. Consider the sequences {x n } { 1/2 1/n } and {y n } { 1/2 1/n }, n 3, then lim f 1 x n,y n lim 4 g x n, lim f 1 y n,x n lim 4 g y n, 2.6 but M f gx n,gy n,gf xn,y n,t which is not convergent to 1 as. Hence the pair f, g is not compatible. t t t f gxn,gy n gf xn,y n t 1/8 1/2 2/n 2, We note that, if f and g are compatible then they are weakly compatible. But the converse need not be true, as shown in the following example. Example Let X, M, be a fuzzy metric space, being a continuous norm with X 2, 20. Define M x, y, t t/ t x y for all t>0, x, y X. Define the maps f : X X X and g : X X by f x, y 2, if x 2orx>5, y X, 6, if 2 <x 5, y X, 2, if x 2, g x 12, if 2 <x 5, x 3, x >

5 Applied Mathematics 5 The only coupled coincidence point of the pair f, g is 2, 2. The mappings f and g are noncompatible, since for the sequences {x n } {y n } {5 1/n }, n 1 we have f x n,y n 2, g x n 2, f y n,x n 2, g y n 2, M f gx n,gy n,g f x n,y n,t t/ t 4 1asn. But they are weakly compatible since they commute at their coupled coincidence point 2, 2. Now we introduce our notions. Aamri and El Moutawakil 8 introduced the concept of E.A. property in a metric space as follows. Let X, d be a metric space. Self mappings f : X X and g : X X are said to satisfy E.A. property if there exists a sequence {x n } X such that lim f x n lim g x n t 2.9 for some t X. Now we extend this notion for a pair of coupled maps as follows. Definition Let X, d be a metric space. Two mappings f : X X X and g : X X are said to satisfy E.A. property if there exists sequences {x n }, {y n } X such that lim f x n,y n lim g x n x, lim f y n,x n lim g y n y, 2.10 for some x, y X. In a similar mode, we state E.A. property for coupled mappings in fuzzy metric spaces as follows. Let X, M, be a FM space. Two maps f : X X X and g : X X satisfy E.A. property if there exists sequences {x n } and {y n } X such that f x n,y n, g x n converges to x and f y n,x n, g y n converges to y in the sense of Definition 2.5. Example Let, be a usual metric space. Define mappings f : X X X and g : X X by f x, y x 2 y 2 and g x 2x for all x, y X. Consider the sequences {x n } {1/n} and {y n } { 1/n}. Since lim f 1 x n,y n lim f lim f y n,x n lim f n, 1 n 1 n, 1 n 1 0 lim g 0 lim g n 1 n lim g x n, lim g y n, 2.11 therefore, f and g satisfy E.A. property, since 0 X.

6 6 Applied Mathematics Remark It is to be noted that property E.A. need not imply compatibility, since in Example 2.12, the maps f and g defined are not compatible, but satisfy property E.A., since for the sequences {x n } { 1/2 1/n } and {x n } { 1/2 1/n } we have lim f 1 x n,y n lim 4 g x n, lim f 1 y n,x n lim 4 g y n, 2.12 since 1/4 X. Recently, Sintunavarat and Kuman 9 introduced a new concept of the common limit in the range of g, CLRg property, as follows. Definition Let X, d be a metric space. Two mappings f : X X and g : X X are said to satisfy CLRg property if there exists a sequence {x n } X such that lim f x n lim g x n g p for some p X. Now we extend this notion for a pair of coupled mappings as follows. Definition Let X, d be a metric space. Two mappings f : X X X and g : X X are said to satisfy CLRg property if there exists sequences {x n }, {y n } X such that lim f x n,y n lim g x n g p, lim f y n,x n lim g y n g q, 2.13 for some p, q X. Similarly, we state CLRg property for coupled mappings in fuzzy metric spaces. Let X, M, be an FM space. Two maps f : X X X and g : X X satisfy CLRg property if there exists sequences {x n }, {y n } X such that f x n,y n,g x n converge to g p and f y n,x n, g y n converge to g q, in the sense of Definition 2.5. Example Let X 0, be a metric space under usual metric. Define mappings f : X X X and g : X X by f x, y x y 2andg x 2 1 x for all x, y X. We consider the sequences {x n } {1 1/n } and {x n } {1 1/n }. Since lim f x n,y n lim f 1 1 n, g 1 lim n lim f y n,x n lim f g 1 1 n, g 1 lim g n therefore, the maps f and g satisfy CLRg property. 1 1 lim n g x n, 1 1 lim g y n, n 2.14 In the next example, we show that the maps satisfying CLRg property need not be continuous, that is, continuity is not the necessary condition for self maps to satisfy CLRg property.

7 Applied Mathematics 7 Example Let X 0, be a metric space under usual metric. Define mappings f : X X X and g : X X by f x, y x y, if x 0, 1, y X, x y, if x 1,, y X, 2 1 x, if x 0, 1, g x x, 2 if x 1, We consider the sequences {x n } {1/n} and {y n } {1 1/n }. Since lim f 1 x n,y n lim f lim f y n,x n lim f n, 1 1 n n, 1 n 1 g 0 lim g g 1 lim g 1 1 n 1 lim n g x n, lim g y n, 2.16 therefore, the maps f and g satisfy CLRg property but the maps are not continuous. We next show that the pair of maps satisfying CLRg property may not be compatible. Example Let X, M, be a fuzzy metric space, being a continuous norm, X 0, 1/2, and M x, y, t t/ t x y for all x, y X and t>0. Define the maps f : X X X and g : X X by f x, y x 2 y 2 /2 and g x x/3, respectively. Consider the sequences {x n } { 1/3 1/n } and {y n } { 1/3 1/n },n>7. Then lim f 1 x n,y n lim 9 g x n, lim f 1 y n,x n lim 9 g y n Further there exists the point 1/3 in X such that g 1/3 1/9, so that the pair f, g satisfies CLRg property. But, M f gx n,gy n,gf xn,y n,t t t f t gx n,gy n gf xn,y n t 1/18 1/9 1/n does not converge to 1 as. Hence, the pair f, g is not compatible.

8 8 Applied Mathematics 3. Main Results For convenience, we denote 1 [ ] n M x, y, t M x, y, t M x, y, t M x, y, t, 3.1 n for all n N. Hu 7 proved the following result. Theorem 3.1. Let X, M, be a complete fuzzy metric space where is a continuous t-norm of H-type. Let f : X X X and g : X X be two mappings and there exists φ Φ such that 2 M f x, y,f u, v,φ t M gx, gu, t M gy,gv,t, 3.2 for all x, y, u, v X and t>0. Suppose that f X X g X, g is continuous, f and g are compatible maps. Then there exists a unique point x X such that x g x f x, x, that is, f and g have a unique common fixed point in X. We now give our main result which provides a generalization of Theorem 3.1. Theorem 3.2. Let X, M, be a Fuzzy Metric Space, being continuous t-norm of H-type. Let f : X X X and g : X X be two mappings and there exists φ Φ satisfying 2 with the following conditions: 3 the pair f, g is weakly compatible, 4 the pair f, g satisfy CLRg property. Then f and g have a coupled coincidence point in X. Moreover, there exists a unique point x X such that x f x, x g x. Proof. Since f and g satisfy CLRg property, there exists sequences {x n } and {y n } in X such that lim f x n,y n lim g x n g p, lim f y n,x n lim g y n g q, 3.3 for some p, q X. Step 1. To show that f and g have a coupled coincidence point. From 2, M f x n,y n,f p, q,t M f xn,y n,f p, q,φ t M gxn,g p,t M gy n,g q,t. 3.4 Taking limit,wegetm g p,f p, q,t 1, that is, f p, q g p x. Similarly, f q, p g q y.

9 Applied Mathematics 9 Since f and g are weakly compatible, so that f p, q g p x say and f q, p g q y say implies gf p, q f g p,g q and gf q, p f g q,g p, thatis,g x f x, y and g y f y, x. Hence f and g have a coupled coincidence point. Step 2. To show that g x x, and g y y. Since is a t-norm of H-type, for any ɛ>0, there exists δ>0 such that 1 δ 1 δ 1 ɛ, }{{} p 3.5 for all p N. Since lim t M x, y, t 1 for all x, y X, there exists t 0 > 0 such that M gx,x,t 0 1 δ, M gy,y,t0 1 δ. 3.6 Also since φ Φ using condition φ 3, we have n 1 φn t 0 <. Then for any t>0, there exists n 0 N such that t> k n 0 φ k t 0.From 2, we have M gx,x,φ t 0 M f x, y,f p, q,φ t 0 M gx,gp,t 0 M gy,gq,t0 M gx,x,t 0 M gy,y,t0, M gy,y,φ t 0 M f y, x,f q, p,φ t 0 M gy,gq,t 0 M gx,gp,t0 3.7 M gy,y,t 0 M gx,x,t0. Similarly, we can also get M gx,x,φ 2 t 0 M f x, y,f p, q,φ 2 t 0 M gx,gp,φ t 0 M gy,gq,φ t 0 M gx,x,φ t 0 M gy,y,φ t 0 [ M ] 2 [ ] 2, gx,x,t 0 M gy,y,t0 M gy,y,φ 2 t 0 M f y, x,f q, p,φ 2 t [ M gy,y,t 0 ] 2 [ M gx,x,t0 ] 2. Continuing in the same way, we can get for all n N, M gx,x,φ n t 0 M gx,x,φ n 1 t 0 M gy,y,φ n 1 t 0 M gx,x,t 0 2 n 1 M gy,y,t0 2 n 1, 3.9 M gy,y,φ n t 0 [ M gy,y,t 0 ] 2 n 1 [ M gx,x,t0 ] 2 n 1.

10 10 Applied Mathematics Then, we have M gx,x,t M gx,x, φ k t 0 k n 0 M gx,x,φ n 0 t 0 [ M gx,x,t 0 ] 2 n 0 1 [ M gy,y,t 0 ] 2 n δ 1 δ 1 ɛ. }{{} 2 n 0 So, for any ɛ>0, we have M gx,x,t 1 ɛ for all t>0. This implies g x x. Similarly, g y y. Step 3. Next we shall show that x y. Since is a t-norm of H-type, for any ɛ>0 there exists δ>0 such that 1 δ 1 δ 1 ɛ, }{{} p 3.11 for all p N. Since lim t M x, y, t 1 for all x, y X, there exists t 0 > 0 such that M x, y, t 0 1 δ. Also since φ Φ, using condition φ-3, we have n 1 φn t 0 <. Then for any t>0, there exists n 0 N such that t> φ k t 0. k n Using condition 2, we have M x, y, φ t 0 M f p, q,f q, p,φ t 0 M gp,gq,t 0 M gq,gp,t0 M x, y, t 0 M y, x, t Continuing in the same way, we can get for all n 0 N, M x, y, φ n t 0 [ M x, y, t 0 ] 2 n 0 1 [ M y, x, t 0 ] 2 n

11 Applied Mathematics 11 Then we have M x, y, t M x, y, φ k t 0 k n 0 M x, y, φ n 0 t 0 [ M x, y, t 0 ] 2 n 0 1 [ M y, x, t 0 ] 2 n δ 1 δ 1 ɛ, }{{} 2 n 0 which implies that x y. Thus, we have proved that f and g have a common fixed point x X. Step 4. We now prove the uniqueness of x.letz be any point in X such that z / x with g z z f z, z. Since is a t-norm of H-type, for any ɛ>0, there exists δ>0 such that 1 δ 1 δ 1 ɛ, }{{} p 3.16 for all p N. Since lim t M x, y, t 1 for all x, y X, there exists t 0 > 0 such that M x, z, t 0 1 δ. Also since φ Φ and using condition φ-3, we have n 1 φn t 0 <. Then for any t>0, there exists n 0 N such that t> φ k t 0. k n Using condition 2, we have M x, z, φ t 0 M f x, x,f z, z,φ t 0 M g x,g z,t 0 M g x,g z,t M x, z, t 0 M x, z, t 0 M x, z, t 0 2. Continuing in the same way, we can get for all n N, M x, z, φ n t 0 M x, z, t 0 2n

12 12 Applied Mathematics Then we have M x, z, t M x, z, φ k t 0 k n 0 M x, z, φ n 0 t 0 M x, z, t 0 2n M x, z, t0 2n δ 1 δ 1 ɛ, }{{} 2 n 0 which implies that x z. Hence f and g have a unique common fixed point in X. Remark 3.3. We still get a unique common fixed point if weakly compatible notion is replaced by w-compatible notion. Now we give another generalization of Theorem 3.1. Corollary 3.4. Let X, M, be a fuzzy metric space where is a continuous t-norm of H-type. Let f : X X X and g : X X be two mappings and there exists φ Φ satisfying 2 and 3 with the following condition: 5 the pair f, g satisfy E.A. property. If g X is a closed subspace of X, thenf and g have a unique common fixed point in X. Proof. Since f and g satisfy E.A. property, there exists sequences {x n } and {y n } in X such that lim f x n,y n lim g x n x, lim f y n,x n lim g y n y, 3.21 for some x, y X. It follows from g X being a closed subspace of X that x g p, y g q for some p, q X and then f and g satisfy the CLRg property. By Theorem 3.2, wegetthatf and g have a unique common fixed point in X. Corollary 3.5. Let X, M, be a fuzzy metric space where is a continuous t-norm of H-type. Let f : X X X and g : X X be two mappings and there exists φ Φ satisfying 2, 3, and 5. Suppose that f X X g X, if range of one of the maps f or g is a closed subspace of X, then f and g have a unique common fixed point in X. Proof. It follows immediately from Corollary 3.5. Taking g I X in Theorem 3.2, thecorollary 3.6 follows immediately the following.

13 Applied Mathematics 13 Corollary 3.6. Let X, M, be a fuzzy metric space where is a continuous t-norm of H-type. Let f : X X X and g : X X be two mappings and there exists φ Φ satisfying the following conditions, for all x, y, u, v X and t > 0: 6 M f x, y,f u, v,φ t M x, u, t M y, v, t, 7 there exists sequences {x n } and {y n } in X such that lim f x n,y n lim x n x, lim f y n,x n lim y n y, 3.22 for some x, y X. Then, there exists a unique z X such that z f z, z. References 1 L. A. Zadeh, Fuzzy sets, Information and Computation, vol. 8, pp , A. George and P. Veeramani, On some results in fuzzy metric spaces, Fuzzy Sets and Systems, vol. 64, no. 3, pp , T. G. Bhaskar and V. Lakshmikantham, Fixed point theorems in partially ordered metric spaces and applications, Nonlinear Analysis, vol. 65, no. 7, pp , V. Lakshmikantham and L. Cirić, Coupled fixed point theorems for nonlinear contractions in partially ordered metric spaces, Nonlinear Analysis, vol. 70, no. 12, pp , S. Sedghi, I. Altun, and N. Shobe, Coupled fixed point theorems for contractions in fuzzy metric spaces, Nonlinear Analysis, vol. 72, no. 3-4, pp , J. X. Fang, Common fixed point theorems of compatible and weakly compatible maps in Menger spaces, Nonlinear Analysis, vol. 71, no. 5-6, pp , X. Q. Hu, Common coupled fixed point theorems for contractive mappings in fuzzy metric spaces, Fixed Point Theory and Applications, vol. 2011, Article ID , 14 pages, M. Aamri and D. El Moutawakil, Some new common fixed point theorems under strict contractive conditions, Mathematical Analysis and Applications, vol. 270, no. 1, pp , W. Sintunavarat and P. Kumam, Common fixed point theorems for a pair of weakly compatible mappings in fuzzy metric spaces, Applied Mathematics, vol. 2011, Article ID , 14 pages, B. Schweizer and A. Sklar, Probabilistic Metric Spaces, vol. 5 of North-Holland Series in Probability and Applied Mathematics, North-Holland, New York, NY, USA, O. Hadžić ande.pap,fixed Point Theory in Probabilistic Metric Spaces, vol. 536 of Mathematics and its Applications, Kluwer Academic Publishers, Dordrecht, The Netherlands, M. Grabiec, Fixed points in fuzzy metric spaces, Fuzzy Sets and Systems, vol. 27, no. 3, pp , M. Abbas, M. A. Khan, and S. Radenović, Common coupled fixed point theorems in cone metric spaces for w-compatible mappings, Applied Mathematics and Computation, vol. 217, no. 1, pp , 2010.

14 Advances in Operations Research Advances in Decision Sciences Mathematical Problems in Engineering Algebra Probability and Statistics The Scientific World Journal International Differential Equations Submit your manuscripts at International Advances in Combinatorics Mathematical Physics Complex Analysis International Mathematics and Mathematical Sciences Stochastic Analysis Abstract and Applied Analysis International Mathematics Discrete Dynamics in Nature and Society Discrete Mathematics Applied Mathematics Function Spaces Optimization